I really don't know if they are, but I would like to know the following. If I post a story with my link on a social bookmarking site, and that post remains on the higher PR page for a good length of time, does this carry as much weight as a website link, blogroll link, header link, etc. for SERP?
Some SN sites are better than others for passing juice, and some pass non at all. Not because of no-follow, but because they are weak in TrustRank and PageRank. Some have been de-valued because of excessive abuse (Squidoo) by link spammers. For one of our sites I remember that materialicious (or something like that) used to show up in our G backlinks, while stumbleupon and digg didn't (until last month's update when many of our backlinks disappeared in G, but our pagerank went from 4 to 6???) Thanks Google.
If you browse the web using firefox, ChromEdit Plus is a great plugin that you can use to have nofollow links automatically highlight.
Nofollow is BS. Comment on blogs and even social networking sites even if they DO have nofollow - Eli from BlueHatSEO.com (very grey hat actually) had this to say in an interview by Aaron Wall of SEOBook.com Read the full interview of Eli by Aaron Wall here: seobook.com/interview-eli-blue-hat-seo-com (I'm new so can't do live links)